1. Field of the Invention
Aspects of the present invention relate to a fuel cell system and a method of operating the same. More particularly, aspects of the present invention relate to a fuel cell system that can rapidly increase an internal temperature of a stack during a start up operation and a method of operating the same.
2. Description of the Related Art
A fuel cell is an electricity generator that changes chemical energy of a fuel into electrical energy through a chemical reaction. A fuel cell can continuously generate electricity as long as fuel is supplied. FIG. 1 is a schematic drawing illustrating the energy transformation structure of a conventional fuel cell. Referring to FIG. 1, when air, which contains oxygen, is supplied to a cathode 1 of a unit cell 10, and a fuel that contains hydrogen is supplied to an anode 3, electricity is generated by a reverse reaction of water electrolysis through an electrolyte membrane 2. However, the electricity generated by a single unit cell typically does not have a high enough voltage for practical use. Therefore, electricity is typically generated by a fuel cell stack 100 in which a plurality of unit cells 10 are connected in series, as depicted in FIG. 2. As depicted in FIG. 3, flow channels including surface flow channels 4a of a bipolar plate 4 for supplying hydrogen or oxygen to the anode and cathode electrodes 1 and 3 are connected in each of the unit cells 10 stacked in the stack 100. Accordingly, when hydrogen and oxygen are supplied to the stack 100 as depicted in FIG. 2, corresponding elements are supplied to the corresponding electrodes and are circulated through the flow channels of each of the unit cells 10.
In an electrochemical reaction, electricity and heat are generated. Therefore, for a smooth operation of a fuel cell, a fuel cell typically must be continuously cooled by dissipating the generated heat. Thus, in the fuel cell stack 100, a cooling plate 5 that passes cooling water for heat exchange is mounted on every 5th or 6th unit cell 10 in the stack 100. Thus, the cooling water absorbs heat in the stack 100 while passing through flow channels 5a of the cooling plate 5. The cooling water that has absorbed heat is cooled in the heat exchanger H5 (refer to FIG. 4) by secondary cooling water, and is circulated back to the stack 100.
In a fuel cell, a hydrocarbon such as a natural gas is used as a fuel source to supply hydrogen to the fuel cell stack 100. Hydrogen is produced from the fuel source in a fuel processor 200, as depicted in FIG. 4, and is supplied to a stack 100.
The fuel processor 200 includes a desulfurizer 210, a reformer 220, a burner 230, a water supply pump 260, first and second heat exchangers H1 and H2, and a carbon monoxide (CO) removing unit 250 consisting of a CO shift reactor 251 and a CO remover 252. The hydrogen generation process is performed in the reformer 220. That is, hydrogen is generated in the reformer 220 heated by the burner 230 through a chemical reaction between a hydrocarbon gas, which is the fuel source entering from a fuel tank 270, and steam that is supplied from a water tank 280 through the water supply pump 260. At this point, CO2 and CO are generated as by products. At this point, the generated CO should be removed, because if a fuel containing 10 ppm or more of CO is supplied to the stack 100, the electrodes are poisoned, resulting in a rapid reduction of the performance of the fuel cell. Therefore, the content of CO in the fuel at an outlet of the reformer 220 is controlled to be 10 ppm or less by passing the reaction products of the reformer 220 through the CO shift reactor 251 and the CO remover 252. A chemical reaction to generate CO2 by reacting CO and steam occurs in the CO shift reactor 251, and an oxidation reaction by directly reacting CO with oxygen occurs in the CO remover 252. The CO content in the fuel that has passed through the CO shift reactor 251 is 5,000 ppm or less and the CO content in the fuel that has passed through the CO remover 252 is reduced to 10 ppm or less. The desulfurizer 210 located at an inlet of the reformer 220 removes sulfur components contained in the fuel source. Sulfur components can easily poison the electrodes of a fuel cell if levels as low as 10 parts per billion (ppb) or more are supplied to the stack 100.
When a fuel cell system having the fuel processor 200 and the stack 100 is operating, hydrogen is generated in the fuel processor 200 through the process described above, and an electrochemical reaction occurs using the hydrogen as a fuel in the stack 100. In FIG. 4, a simplified stack 100 is depicted. However, as described with reference to FIG. 3, hydrogen passes through corresponding flow channels to contact anode electrodes. Air, which is a source of oxygen, passes through corresponding flow channels to contact cathode electrodes. As shown in FIG. 4, a process burner 110 is operated using surplus hydrogen that is not consumed in the stack 100. Secondary cooling water that has heat exchanged with the cooling water that is circulating through the stack 100 can be sent to the water storage for use as warm water. However, the temperature of the secondary cooling water is not high enough to be a significant source of warm water. Therefore, recently, a fuel cell system having a structure in which the process burner 110 that uses surplus hydrogen has been generally employed. The process burner 110 heats water, and the heated water is stored in a warm water storage 120 for using warm water for extraneous uses.
In order to have a efficient electrochemical reaction in the stack 100, the interior of the stack 100 must be maintained at an appropriate temperature. For example, a normal operating temperature of the stack 100 may be 120° C. However, during a start up operation of a fuel cell system, it takes time for the stack 100 to reach the normal operating temperature. During the start up operation of a fuel cell system, to increase the temperature of the stack 100, a cooling water tank 130 may be heated using an electric heater, so that the temperature of the stack 100 may be increased by circulating the heated water. Of course, when the stack 100 reaches the normal temperature and the normal electrochemical reaction begins to occur, the temperature of the stack 100 rises due to an exothermic reaction. But, during the start up operation, the interior of the stack 100 is heated to an appropriate temperature by circulating heated cooling water using an electric heater. However, when the stack 100 is heated using the heated cooling water, it takes approximately one hour to heat the stack 100 from room temperature to the normal operating temperature of, for example, 120° C. Accordingly, although the fuel processor 200 may ready to supply hydrogen to the stack 100 shortly after start up, it may not be possible to begin the operation of the fuel cell stack 100 until one hour later, when the temperature of the stack 100 has reached a normal operating temperature.
Accordingly, there is a need to develop a system that can rapidly heat the stack 100 during the start up operation in order to reduce the time required to reach a normal operation of a fuel cell system.